Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	$ 22,577	$ 22,153
60-89 Days Past Due	1,780	3,358
Greater Than 90 Days	2,319	2,573
Total Past Due	26,676	28,084
Total Current	2,661,612	2,656,201
Total	2,688,288	2,684,285	2,128,015
Total 90 Days Past Due Still Accruing	30	133	24
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	3,903	5,303
60-89 Days Past Due	187	287
Greater Than 90 Days	177	420
Total Past Due	4,267	6,010
Total Current	603,014	590,720
Total	607,281	596,730
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	535	355
60-89 Days Past Due	58
Greater Than 90 Days	5
Total Past Due	598	355
Total Current	64,523	75,573
Total	65,121	75,928
Total 90 Days Past Due Still Accruing	4
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	16,515	13,787
60-89 Days Past Due	1,192	2,489
Greater Than 90 Days	1,991	1,876
Total Past Due	19,698	18,152
Total Current	1,661,109	1,660,362
Total	1,680,807	1,678,514
Total 90 Days Past Due Still Accruing	11	55
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	1,624	2,708
60-89 Days Past Due	343	582
Greater Than 90 Days	146	277
Total Past Due	2,113	3,567
Total Current	332,966	329,546
Total	335,079	333,113
Total 90 Days Past Due Still Accruing	$ 15	$ 78